Aqueous recording liquids (hereinafter often referred to simply as “recording liquids”) containing a water-soluble dye as a colorant have been used as recording liquids to be mounted in ink-jet printers, in which a recording liquid is ejected as minute droplets from a thin nozzle to a recording paper to record an image, or in other printers. However, water-soluble dyes have had a drawback that when the recording liquids are printed on materials to be recorded, e.g., recording papers, the printed image has insufficient water resistance. There also has been a problem that the printed matters deteriorate due to the alteration of the dyes by light (problem concerning light resistance). In order to eliminate such problems, investigations have been made on recording liquids containing as a colorant a pigment excellent in water resistance and light resistance. For example, with respect to black recording liquids, carbon blacks have been investigated as pigments therefor.
In order for a carbon black to be used in a recording liquid for ink-jet printers or the like, the carbon black should be dispersed in water. However, carbon blacks in themselves are difficult to disperse in water or undergo or cause aggregation, sedimentation, and/or gelation after dispersion. It is therefore necessary to physically adsorb a compound having surface activity onto the surface of a carbon black or to chemically treat the surface of a carbon black to sufficiently hydrophilize the surface of the carbon black.
On the other hand, in order for such an aqueous recording liquid to give a clear printed matter having a high blackness, it is important that after the recording liquid is ejected onto a material to be recorded, the water and other ingredients constituting the dispersion medium should be rapidly absorbed into the material to be recorded itself or into the absorbing layer of the material to be recorded, as different from the case of ordinary printing inks containing a polymer resin or the like as the main component. This is because in case where the dispersion medium of the recording liquid remains on the surface of the material to be recorded for a long time, a prolonged drying time is necessary and this may result in blurring or in fouling by contact with printed areas. Consequently, for reducing the drying time, it is necessary to facilitate the separation of the carbon black from the dispersion medium of the recording liquid on the surface of the material to be recorded.
Furthermore, the aqueous recording liquid is required to have a sufficiently low viscosity so as to be prevented from clogging the thin nozzle of a printer when ejected therefrom and be easily ejected from the nozzle. In addition, the recording liquid is required to be prevented from suffering carbon black sedimentation or the like during the storage of the recording liquid. It is therefore necessary that the particle diameters of the carbon black dispersed in the recording liquid, i.e., the particle diameter of primary aggregates (structures) each made up of primary carbon black particles gathered together and that of secondary aggregates each made up of such primary aggregates gathered together, should be sufficiently small and that the amount of the carbon black contained in the recording liquid should be minimized.
However, the recording liquid is further required to have the following property. After the recording liquid is printed on a porous material to be recorded, e.g., paper, carbon black infiltration into the material to be recorded is diminished so that the amount of carbon black particles remaining on the surface of the material to be recorded becomes as large as possible to thereby give a printed matter having a high black color density. For attaining this, it is necessary to use a carbon black having a large particle diameter as compared with the surface pores of the material to be recorded. It is also known that to increase the carbon black content in a recording liquid and to increase the viscosity of a recording liquid are effective.
As described above, in the recording liquids, the attainment of satisfactory storage stability or suitability for ejection from a printer or the like and the formation of a clear print having high blackness on a material to be recorded are antinomic subjects. There has been a problem that print quality such as blackness should be sacrificed because an improvement in the suitability for ejection of a recording liquid currently has priority for the purpose of, e.g., diminishing troubles in apparatus such as printers.
Under these circumstances, use of a carbon black having a large structure diameter is being investigated in order to improve the blackness of printed matters.
For example, the following have been proposed as ink-jet recording liquids attaining a high print color density: a recording liquid containing a carbon black which has a primary-particle diameter (D) of 25–120 nm, DBP absorption (B) of 40–180 cm3/100 g, and pH of 2–6 and in which the particle diameter (D) and the DBP absorption (B) satisfy the relationship 10D+B≧400 (see patent document 1); a recording liquid containing a carbon black having a DBP absorption of 140 cm3/100 g or higher and a volatile content of 4% by weight or lower (see patent document 2); and a recording liquid containing a carbon black having a DBP absorption of 140 cm3/100 g or higher and a pH of 6 or higher and a water-soluble high-molecular compound having an acid value of 250 mgKOH/g or higher (see patent document 3).
In general, a carbon black having a large aggregate diameter tends to come to have a widened aggregate diameter distribution or an increased proportion of carbon black aggregates having a large diameter in the course of structure growth in a production step, although these tendencies depend on the production process used. Consequently, when such a carbon black having a large aggregate diameter is used as a black pigment for a recording liquid, there are cases where ink-jet nozzle clogging occurs. Because of this, use of a carbon black having a narrow aggregate particle diameter is also being investigated.
For example, the following have been proposed as recording liquids giving a printed matter having a high blackness (having a high print color density): a recording liquid containing a carbon black having a D1/2/Dmod of 0.53 or lower (see patent document 4); and a recording liquid containing a carbon black having a small particle diameter, small aggregate diameter, and narrow aggregate diameter distribution (a carbon black having an N2 SA of 200 m2/g or larger, Dmod of 80 nm or smaller, D1/2/Dmod ratio of 0.6 or lower, and pH of 5 or higher) as a carbon black having satisfactory dispersibility (see patent document 5).
Furthermore, use of a carbon black having a low volatile content and reduced in the amount of surface functional groups is also being investigated in order to improve the dispersibility of a carbon black in a recording liquid. For example, recording liquids have been proposed which employ a neutral carbon black and an alkaline carbon black and are thereby satisfactory in storability and suitability for ejection (see patent documents 6 and 7).
[Patent Document 1] JP-A-7-316480
[Patent Document 2] JP-A-10-17808
[Patent Document 3] JP-A-2000-290554
[Patent Document 4] JP-A-10-168371
[Patent Document 5] JP-A-11-335583
[Patent Document 6] JP-A-10-81842
[Patent Document 7] JP-A-10-17808